In the production of fuel and synthesis gases from carbonaceous or hydrocarbon raw materials, e.g. coal or coal-like solids, petroleum and petroleum byproducts, refinery or residues and products and the like, the fuel or starting material is subjected to gasification to produce a product which may be rich in one or more of the components: carbon monoxide, methane and hydrogen.
Such gases may be used as fuels or industrial, commercial and residual purposes or may be used as synthesis gases for the production of more complex chemical gases. In the former case, the presence of sulfur or sulfur-containing contaminants give rise to substantial environmental pollution problems while, in the second case, the presence of even small amounts of sulfur or sulfur-containing contaminants may poison or otherwise act detrimentally upon the chemical-synthesis catalyst. In both cases, the presence of sulfur-containing impurities give rise to an increase in corrosion of conduits and equipment for processing or utilizing the gas.
The sulfur compounds in raw fuel gases and synthesis gases consist primarily of hydrogen sulfide, various sulfur-containing organic compounds of the mercaptan type, other sulfides such as carbon disulfides and organic sulfides, and carbon oxysulfide. Carbon disulfide is a significant contaminant when the gases are produced by the coking or gasification of coal.
Only a small part of the carbon oxysulfide contained in such gases can be absorbed by the absorbent solutions hitherto used for the scrubbing of such gases to remove the other sulfur-containing impurities. Consequently, where sulfur contamination is a critical problem, as in the case of synthesis gases to be used in conjunction with sulfur-sensitive catalysts, a fine purification step must be provided to recover the non-absorbent carbon oxysulfide. For this purpose the gas is passed into contact with zinc oxide or iron oxide at temperatures of 300.degree. to 350.degree.C and upon which the carbon oxysulfide is absorbed.
While this process is effective to remove the carbon oxysulfide, it is too expensive to be practical for many industrial purposes since renewal of adsorbent must be carried out frequently, the reaction requires inordinately high temperatures and therefore large amounts of energy, and because the apparatus necessary for carrying out the fine-purification step is relatively expensive. In practice, the use of adsorbent stages of this type has been confined to gases which have been prepurified to a significant degree.
It is also known to remove sulfur-containing impurities from gas streams by chemical action, e.g. by neutralization or chemical-bonded-forming techniques. In these systems, the adsorbents are aqueous or water-containing solutions of strong organic bases or alkali solids of weak inorganic or organic acids. The absorption capacity of such solutions depends upon their alkaline reaction with the carbon-containing contaminants and hence upon the stoichiometry of the reaction. The absorption is most effective with acidic sulfur-containing components such as H.sub.2 S and other acidic components such as carbon dioxide present in the gas stream. The absorbent solution is regenerated at atmospheric pressure by heating and the use of a stripping gas. The carbon oxysulfide, however, is substantially uneffected by the absorption process since it is chemically inert to the absorbent under the ambient conditions under which the absorption normally takes place.
It is also known to utilize a physical scrubbing process whereby the gas is treated at a pressure of at least 10 kg/cm.sup.2, with an absorbent consisting of neutral organic liquid compounds to which water may be added. Unlike the chemisorption process previously described, the absorption capacity of the organic compounds serving as absorbents does not depend upon the stoichiometry of a chemical reaction but rather of the specific solubilities of the various gas components and, therefore, upon the absorption coefficient or .alpha. value. The absorption is also dependent upon the partial pressures of the various gas components and the total pressure. The absorbents are regenerated by flashing to atmospheric pressure and stripping with an inert gas with or without heating.
Because of its low solubility, carbon oxysulfide is generally unsatisfactorily absorbed using physical processes and, indeed, significant absorption only occurs when high flow rates of the solvent are employed and substantial absorption of carbon dioxide can be tolerated.
Because of these problems in the removal of carbon oxysulfide from fuel and synthesis gases, various techniques have been developed which are intended to specifically remove carbon oxysulfide or render the same removable. For example, it has been proposed to react aliphatic amines with carbon oxysulfide to form thiocarbamates and thereby remove the carbon oxysulfide from the gas. The aliphatic amines are generally alkanolamines which, when reacted with carbon oxysulfide are not readily reformed or regenerated. Thus complex regeneration systems have been provided in conjunction with these arrangements, e.g. by treating the alkanolamine/carbon oxysulfide system with water, heating the system and stripping it with a gas. This technique has the obvious disadvantage that it requires a number of steps and considerable equipment for carrying out the process.
It has also been suggested that the gas may be treated in three stages using in the first stage an aqueous solution of ethanolamine hydrolyzing the carbon oxysulfide at a temperature above 60.degree.C to hydrogen sulfide and carbon dioxide, and thereafter scrubbing the hydrolyzed system. The disadvantage with this arrangement, as in the previous case, is that the reaction of carbon oxysulfide and water to yield hydrogen sulfide and carbon dioxide normally favors the decomposition of the carbon oxysulfide although the reaction rate is relatively slow in ambient temperatures. Thus higher temperatures may be required than are otherwise desired. Of even greater significance is the fact that the mass action relationship may be displaced in the opposite direction when the gas contains large quantities of carbon dioxide and hydrogen sulfide and may have a low moisture content. In this case, even concentrations up to about 10% carbon dioxide and hydrogen sulfide result in intolerable concentrations of carbon oxysulfide.
As with the previously described chemisorption processes, the last-mentioned system also has the disadvantage that the acidic gas components hydrogen sulfide and carbon dioxide are jointly recovered and hence that the sulfur-containing product is contaminated with large quantities of carbon dioxide. It is difficult to convert the sulfur compounds to nonvolatile elemental sulfur and sulfuric acid and hence to utilize the sulfur-containing components.